Nonaqueous electrolyte secondary batteries, such as a lithium secondary battery, are currently in wide use as batteries for devices such as a personal computer, a mobile telephone, and a portable information terminal.
Such a nonaqueous electrolyte secondary battery, typified by a lithium secondary battery, has a high energy density and may thus let a large current flow and generate heat in a case where a breakage in the battery or in a device using that battery has caused an internal or external short circuit. This risk has created a demand that a nonaqueous electrolyte secondary battery should prevent more than a certain level of heat generation, so as to ensure a high level of safety.
Safety of a nonaqueous electrolyte secondary battery is typically ensured by imparting a shutdown function to a nonaqueous electrolyte secondary battery. The shutdown function blocks passage of ions between a cathode and an anode with use of a separator so that further heat generation will be prevented, in a case where abnormal heat generation has occurred. More specifically, a nonaqueous electrolyte secondary battery typically includes a separator between a cathode and an anode, which separator has a function of, in a case where, for example, an internal short circuit between the cathode and the anode has caused an abnormal current to flow through the battery, blocking that current and preventing a flow of an excessively large current (shutdown) so that further heat generation will be prevented. The shutdown is performed such that in a case where a nonaqueous electrolyte secondary battery has been heated to a temperature over a normal operating temperature, a separator is melted by heat and thereby pores of the separator are clogged. It is preferable that after the shutdown, the separator not be broken by heat even in a case where a temperature inside the battery has reached a relatively high temperature, but maintain a shutdown state.
The separator is typically made of a porous film that contains, as a main component, a polyolefin, which melts at, for example, approximately 80° C. to 180° C. in a case where abnormal heat generation has occurred. However, a separator made of such a porous film has insufficient shape stability at high temperatures. This poses a risk that even in a case where the shutdown function is performed, the occurrence of shrinkage, breakage of the film, or the like may bring a cathode and an anode into direct contact with each other, and may consequently lead to an internal short circuit. That is, a separator made of the porous film may not be able to sufficiently prevent abnormal heat generation caused by an internal short circuit. This risk has created a demand for separators that are capable of ensuring a higher level of safety.
For example, Patent Literature 1 proposes, as a porous film having an excellent heat resistance, a porous film including a polyolefin microporous film and a heat-resistant porous layer which is made of an aromatic polymer such as aromatic polyamide and is stacked on the polyolefin microporous film.
A nonaqueous electrolyte secondary battery, such as a lithium secondary battery, is a battery that may burn because it contains a flammable organic electrolyte. Thus, in view of improper use by a user and use under a severe environment, various safety standards have been developed and various safety tests have been performed.
Examples of such safety standards include an IEC standard (developed by the International Electrotechnical Commission), a UL standard (developed by Underwriters Laboratories Inc.), and a JIS standard.
Examples of the safety tests include an electrical test (external short-circuit test, overcharging test, etc.), an environmental test (low-pressure test, heating test, etc.), and a mechanical test (crush test, drop test, etc.).
More specifically, a nail penetration test is an example of a test for evaluating an internal short circuit, which is a possible cause of a lithium-ion battery accident. In the nail penetration test, a nail is caused to penetrate a battery so as to artificially cause an internal short circuit, and safety of the battery (e.g., presence of heat generation, ignition, smoking, and bursting) is evaluated.